1. Field of the Invention
The present invention is directed to a metal halide discharge lamp, and more particularly a discharge lamp having an arc tube filled with metal halides.
2. Description of the Prior Art
Metal halide discharge lamps have been used in a wide variety of fields because of its superior performances, such as high luminance, high efficiency, and high color rendering properly. Among these, a metal halide lamp having an arc tube filled with sodium halide and scandium halide is preferred as it shows a less color change. That is, even when luminous intensity of reddish color from vapors of sodium halide varies to some extent, vapor of the scandium halide can provide a continuous color spectrum, thereby giving less change in color. Such discharge lamp is disclosed in the following listed prior art.
List of the Prior Art
a) Japanese Patent Early Publication No. 6-84496
b) Japanese Patent Early Publication No. 6-111772
c) Japanese Patent Early Publication No. 8-203471
d) Japanese Patent Early Publication No. 55-32355
e) Japanese Patent Early Publication No. 56-109447
Concise Explanation of the Listed Prior Art
Publication No. 6-84496 and No. 6-111772 disclose a metal halide lamp having an arc tube filled with sodium iodide, scandium iodide, and an inert gas but without mercury. It is described in this publication that due to the absence of mercury, color spectrum is substantially the same irrespective of a variation of an input power, causing no substantial change in color.
Publication No. 8-203471 discloses a metal halide lamp having an arc tube filled with sodium iodide scandium iodide, and a xenon gas. The arc tube is sealed within an envelope which is evacuated or filled with a lower pressure gas for thermally insulating the arc tube from outside of the envelope for limiting a cooling effect of the arc tube.
Publication No. 55-32355 discloses a metal halide lamp having an arc tube filled with sodium iodide, scandium iodide, mercury, and an inert gas. Scandium iodide is filled in a specific range of amount in relation to a rated lamp power, while a ratio of the filling amount of sodium iodide to that of scandium iodide is selected to a specific value, in order to improve lamp efficiency and operational life period.
Publication No. 56-109447 discloses a metal halide lamp having an arc tube filled with sodium iodide, scandium iodide, mercury, and an inert gas. The lamp is designed to satisfy a specific range as to a molar ratio of sodium iodide to scandium iodide, and at the same time to satisfy a specific relation between the molar ratio and cold spot temperature during a normal lamp operation at a rated power.
Problem of the Prior Art
However, the prior art discharge lamp is found still insufficient in keeping a uniform color when subjected to variations in a lamp power as well as in a voltage supplied to the lamp. Thus, dimming control of varying the lamp power may result in undesired color change of the lamp, and Thus, undesired color change may occur when dimming the lamp by varying the lamp power or when there is a variation in an output voltage from a ballast as a result of a variation in the line voltage, or in quality of the ballast, or even in quality of the lamp.
In view of the above, the present invention has been achieved to provide a metal halide discharge lamp which is capable of reducing a color change when subjected to a variation in the lamp power and/or the voltage supplied to the lamp. The metal halide lamp in accordance with a present invention comprises an arc tube filled with at least sodium halide and scandium halide. The arc tube is formed at its opposite ends with electrodes which gives an arc discharge therebetween. The lamp has regulator means for keeping a coldest spot temperature of the arc tube at 550xc2x0 C. or more when operating the lamp at a lamp power which is 50% of rated lamp power. It is found that when the lamp is configured to have a coldest spot temperature at 550xc2x0 C. or more when operating the lamp at a lamp power which is 50% of the rated lamp power, the lamp shows much less color change even subjected to the lamp voltage variation, thereby maintaining a desired color. The arc tube may be made of quartz or a transparent ceramic.
The lamp includes an envelope which forms a hermetically sealed space for accommodating therein the arc tube. The envelope is evacuated or filled with low pressure inert gas to define the regulator means. The envelope may be coated on its inner surface with a layer of reflecting an infrared radiation or with a phosphor.
Preferably, scandium halide is filled the arc tube in an amount of less than 4.08 mol/mlxc3x9710xe2x88x926 mol/ml to stabilize the arc discharge.
In a preferred embodiment, the lamp include a sleeve surrounding the arc tube to reduce a heat loss form the arc tube. Thus, the sleeve defines the regulator means alone or in combination with the envelope. The sleeve may be coated on its inner surface with a layer of reflecting an infrared radiation. The layer may be coated on the entire surface or partially on opposite ends of the sleeve corresponding to the electrodes.
Further, the lamp includes heat insulators formed on the arc tube at portions covering the respective electrodes so as to thermally insulate the portions of the arc tube adjacent the electrodes from the outside thereof. Thus, the heat insulators can define the regulator means alone or in combination with the envelope or the sleeve. The heat insulator may be a metal layer of reflecting the infrared radiation.
The arc tube may be formed to have reduced-in-diameter sections at opposite ends of the tube which have a diameter less than the rest and surround the electrodes, respectively. With the provision of the reduced-in-diameter sections, the opposite ends of the arc tube is kept at a relatively high temperature due to the heat from the adjacent electrodes. Thus, the sections can define the regulator means alone or in combination with the envelope, sleeves, or the heat insulators.
Formed at opposite ends of the arc tube are sealed ends for sealing the electrodes. The sealed ends are preferably made to have an outside diameter less than that of the arc tube for retarding the cooling of the arc tube around the electrodes. Thus, the sealed ends can also define the regulator means.
A molar ratio (R) of sodium halide to scandium halide is preferably between 2.8 to 22.7 in order to reduce color change when the lamp subjected to the variation in the voltage supplied to the lamp. For the lamp having a rated lamp power of less than 400 W, the molar ratio is preferably between 2.8 to 17.0. For the lamp having a rated power of 400W or more, the molar ratio is preferably between 5.7 to 22.7. The arc tube may additionally include cesium iodide or mercury.
For one lamp configuration where the envelope is evacuated, and the arc tube is made of quartz into a cylindrical shape and is formed on opposite ends with the heat insulators covering the electrodes, the arc tube is preferably designed to have an inside diameter of about 8 mm and a distance of about 80 mm between the electrodes, and is filled with about 2.32xc3x9710xe2x88x925 mol/ml of sodium iodide, about 2.04xc3x9710xe2x88x926 mol/ml of scandium iodide, about 1.2xc3x9710xe2x88x925 mol/ml of cesium iodide, and about 27000 Pa of xenon.
For another lamp configuration where the envelope is evacuated with its inner surface coated with a phosphor layer, and the arc tube is made of quartz into a cylindrical shape and is formed on opposite ends with the heat insulators covering the electrodes, the arc tube is preferably designed to have an inside diameter of about 8 mm and a distance of about 80 mm between the electrodes, and is filled with about 2.32xc3x9710xe2x88x925 mol/ml of sodium iodide, about 2.04xc3x9710xe2x88x926 mol/ml of scandium iodide, about 2.5xc3x9710xe2x88x925 mol/ml of mercury and about 6700 Pa of argon.
For a further lamp configuration where the arc tube is made of quartz into a ellipsoidal shape and is formed on opposite ends with the heat insulators covering the electrodes and with sealing ends for sealing the electrodes, and the correspondingly shaped envelope is evacuated, the ellipsoidal arc tube is preferably designed to have a maximum inside diameter of about 18 mm, an average inside diameter of about 14 mm, and a distance of about 48 mm between the electrodes, and is filled with about 1.35xc3x9710xe2x88x925 mol/ml of sodium iodide, about 1.15xc3x9710xe2x88x928 mol/ml of scandium iodide, about 2.14xc3x9710xe2x88x925 mol/ml of mercury and about 6700 Pa of argon. In this configuration, the sealed ends are also designed to be smaller in diameter than the arc tube.
For a still further lamp configuration where the arc tube is made of quartz into a ellipsoidal shape and is formed on opposite ends with the heat insulators covering the electrodes and with sealing ends for sealing the electrodes, and the correspondingly shaped envelope is evacuated, the ellipsoidal arc tube is preferably designed to have a maximum inside diameter of about 18 mm, an average inside diameter of about 14 mm, and a distance of about 48 mm between the electrodes, and is filled with about 1.35xc3x9710xe2x88x925 mol/ml of sodium iodide, about 1.15xc3x9710xe2x88x926 mol/ml of scandium iodide, and about 6700 Pa of argon, said envelope being filled with about 47000 Pa of nitrogen gas. Also in this configuration, the sealed ends are also designed to be smaller in diameter than the arc tube.
These lamp configurations are particularly advantageous for realizing the regulator means for maintaining the coldest spot temperature of the arc tube at 550xc2x0 C. or more when operating the lamp at a lamp power which is 50% of rated lamp power, thereby reducing the color change even subjected to the variation in the voltage supplied to the lamp.
These and still other objects and advantageous features of the present invention will become more apparent from the following description of the embodiments when taken in conjunction with the attached drawings.